1. Field of the Invention
This invention relates to marine seismic surveying, and is more particularly concerned with noise reduction methods and apparatus for use in marine seismic survey.
2. Description of Related Art
In order to perform a 3D marine seismic survey, a plurality of seismic streamers, each typically several thousand metres long and containing arrays of hydrophones and associated electronic equipment distributed along its length, and towed at about 5 knots behind a seismic survey vessel, which also tows one or more seismic sources, typically air guns. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones in the streamers, digitized and then transmitted to the seismic survey vessel, where they are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed.
Typically up to 12 streamers are towed, each streamer being several kilometers long. The streamers are made up of sections which may be typically 100-200 meters long; each section consists of hydrophones inside an outer skin which may be filled with oil, foam, or a more solid substance. Sress-wires and spacers for the internal skeleton of the streamer.
The signals received by the hydrophones in the streamers are invariably contaminated by noise from a variety of sources. The lurching of the vessel, especially in rough seas, causes vibrations in the stress-wires which interact with the connectors and the oil-filled skin, generating bulge waves (or breathing waves) which propagate down the streamers. The pressure variations are detected by the hydrophones, adding noise and corrupting the detected seismic signals. As the streamer moves through the water, boundary layer turbulence causes pressure fluctuations at the outer skin wall, which are again coupled to the hydrophones.
Bulge waves may also be caused by eddy shedding under elliptical water motion about the streamer caused by wave action. Currently, one of the main techniques used to reduce this noise involves hard-wiring groups of adjacent hydrophones together, to sum their respective analogue output signals: typically, a group contains eight uniformly-spaced hydrophones, and the centres of the groups are typically spaced at 6.25 meter intervals. Such an arrangement is disclosed in our U.S. Pat. No. 5,351,218, which also describes how pairs of adjacent groups can be selectively connected together to form groups of sixteen adjacent hydrophones whose group centres are spaced at 12.5 meter intervals.
Since the individual hydrophones in each group are fairly closely spaced, at typically just under 90 cm apart, it is assumed that all the hydrophones in a given group receive substantially the same seismic signal. The seismic signal is therefore reinforced by the summing of the analogue output signals of the hydrophones of the group, while the noise affecting each hydrophone, if it is randomly uncorrelated, will tend to be cancelled out by the summing process. The groups of eight or sixteen hydrophones can thus be considered equivalent to single hydrophones with a 6.25 or 12.5 meter spacing, a gain of eight or sixteen in relation to an individual hydrophone within a group, and providing quite good rejection of random noise.
However, a significant source of the noise affecting the hydrophones is the motion of the surface of the water in the area of the survey due to waves and swell. Noise due to waves or swell, which will hereinafter be referred to simply as xe2x80x9cswell noisexe2x80x9d, is not truly random in relation to the groups of eight or sixteen hydrophones of the prior art, so that the summing of the analogue output signals of the hydrophones in each group is not very effective in reducing it. Furthermore, the significance of swell noise increases dramatically as the height of the waves or swell increases, to the extent that when the weather causes the height to exceed a certain level, typically 2 to 4 meters, the signal-to-noise ratio deteriorates so much that the survey has to be suspended until the weather improves. This xe2x80x9cweather downtimexe2x80x9d can add substantially to the overall cost of the survey.
A method of applying adaptive signal processing to the attenuation of bulge waves is described U.S. Pat. No. 4,821,241. There it is proposed to co-locate stress sensors with the hydrophones in the streamer. The stress sensors are responsive to mechanical stresses applied to the cable, but are substantially unresponsive to acoustic waves propagating in fluid media. The signal outputs from the stress sensors are combined with the signal outputs from the corresponding co-located hydrophones to cancel spurious signals due to bulge waves.
Another method of applying adaptive signal processing to the attenuation of bulge waves is described U.S. Pat. No. 5,251,183. In this patent it is proposed to use an accelerometer secured between the lead-in section of the streamer and the hydrophone. Intra-shot and inter-shot accelerometer and hydrophone signals are recorded. The method utilizes inter-shot and intra-shot adaptive processing loops. The inter-shot adaptive processing loop derives inter-shot complex weights from inter-shot accelerometer signals and inter-shot hydrophone signals. The intra-shot adaptive processing loop models bulge wave noise in the intra-shot hydrophone signals by combining the inter-shot complex weights with intra-shot accelerometer signals. Bulge wave noise attenuation is achieved by subtracting the intra-shot bulge wave noise model from the intra-shot seismic detector signals.
Other types of noise, such as crossflow noise generated when the streamer is subjected to cross-currents, have characteristics that are similar to the characteristics of this bulge wave noise. Crossflow noise is a particularly significant problem when the seismic survey vessel and streamers are being turned during a seismic survey. The level of crossflow noise is typically so overwhelming during turns that seismic data recording is simply stopped while the vessel is being turned. Prior art seismic data signal processing methods have failed to adequately attenuate these types of noise as well.
It is therefore an object of the present invention to provide methods and apparatus for reducing the effects of noise, such as swell noise, in marine seismic surveys.
According to one aspect of the present invention, there is provided a seismic acquisition system comprising: a streamer for receiving seismic signals reflected from strata beneath a body of water, the streamer comprising a plurality of hydrophones distributed at average intervals of not more than 500 cm therealong; analogue-to-digital converter means receiving and digitizing analogue signals of the hydrophones to generate for each hydrophone a separate digitized signal; and a filter receiving as input the digitized signal together with the digitized signal of at least one nearby hydrophone and generating a signal with reduced noise content.
According to another aspect of the present invention, there is provided a method of performing a marine seismic survey, the method comprising:
towing at least one seismic streamer comprising a plurality of hydrophones distributed at average intervals of not more than 500 cm therealong in the water over the area to be surveyed;
directing acoustic signals down through the water and into the earth beneath;
receiving with the hydrophones seismic signals reflected from strata in the earth beneath the water;
digitizing an output of each hydrophone separately; and
filtering said output to reduce the noise present in said output and to generate a signal with a reduced noise content, said filtering step using as a further input the digitized output of at least one nearby hydrophone.
The invention is based on the discovery that the dominant noise components typically have a coherence length of less than 20 meters in the low frequency range. By sampling the wavefield at intervals significantly below that distance to avoid or at least reduce aliasing, and applying an appropriate noise filtering technique, it is possible to reduce the amount of noise in recorded streamer data more efficiently than by known group forming methods. The coherence length of the noise is determined by various parameters, including streamer design, construction, towing and weather conditions.
Specifically, the invention proposes sampling of the wavefield at an average sampling distance of less than 500 cm. This sampling density cannot be achieved by conventional xe2x80x9cgroup-formedxe2x80x9d acquisition data, where the output of adjacent hydrophones are wired together. This known technique averages the measurement over the coherence length of the noise, thus rendering it unsuitable for conventional coherent noise filtering methods.
The invention provides a system which can convert the seismic signal as measured by a single hydrophone into a noise-reduced version of the same signal. This noise-reduced signal can be available to subsequent processing steps either in the so-called xe2x80x9cpre-stackxe2x80x9d domain or the signal can be combined with the noise-reduced signals of other hydrophones in a process commonly referred to as xe2x80x9cstackingxe2x80x9d.
The term xe2x80x9cadjacent hydrophonesxe2x80x9d is meant to include directly neighboring hydrophones, but also cases where the inputs of nearby (but not directly adjacent) hydrophones are used. Preferably, the hydrophone spacing lies in the range 200 cm to 330 cm, and may for example be about 205 cm to 210 cm, or about 305 cm to 315 cm.
Spatio-temporal filtering is preferably applied to attenuate noise from the received signals. Generally it is possible to broadly distinguish in seismic surveys between noise and seismic signal in terms of frequency, direction of propagation and (apparent) velocity.
In accordance with another aspect of the present invention, the filter acts as a beamformer, i.e. discriminating its response in accordance with the spatial and/or temporal spectral content of the input signals.
Preferably the filter is an adaptive filter, even more preferably a filter adapting its filter weights or coefficients under a predefined set of constraints.
In a particularly preferred embodiment the filter comprises M spatially and/or temporally local multichannel adaptive filters with K channels, each of a length L. For most applications, the numbers M, K and L are equal to or larger than two.
The use of a filter bank for noise attenuation of seismic signals has been described in International Patent Application No. WO97/25632. However, the present invention does not require defining a reference channel in order to calculate the adapted filter bank coefficients. In other words, no noise estimate enters the adaptation process. Therefore, the present method can be applied to noise contaminated seismic signals, where there is no independent measurement or estimate of the noise available.
According to one aspect of the invention, the coefficients of the filter bank are constrained such that its response corresponds to that of a beamformer with a specified look-direction.
The method can be performed on stored data or on raw seismic data as it is acquired. Thus raw seismic data may be filtered according to the method at the data acquisition site. This ensures that a xe2x80x9ccleanedxe2x80x9d signal is available from the data acquisition site and may be downloaded directly from the site in this form. This reduces the amount of data that must be sent for analysis off-site and reduces the costs and storage problems associated with accumulating sufficient quantities of noisy data for analysis off-site. The method can be applied to single-sensor recordings, i.e. to recordings prior to any group forming which combines the signals of two or more seismic sensors.